The present invention relates generally to the injection molding of plastics and, more particularly, to a process for injection molding a hollow plastic article wherein a low pressure heat-activated gas is combined with a plastic material during injection of the plastic material into a mold cavity of a plastic injection mold unit.
It is well known in the prior art such as U.S. Pat. No. 4,101,617 to combine a highly compressed gas such as nitrogen in a mold cavity with injected plastic material in order to expand and distribute the plastic material against the walls of the mold cavity thereby forming a hollow portion therein which is filled with the highly pressurized gas. Most of the prior art processes involve partially filling the mold cavity with a plastic material and then injecting the gas into the mold cavity. The gas snakes its way through the thickest sections which are still molten and thus offer the least resistance. It is at these sections where the hollow portions are formed.
However, these high pressure gases such as nitrogen have many drawbacks. As a molded plastic article cools and hardens, the plastic material shrinks. The expanded gases of the prior art are cold and therefore provide a chilling effect to the inside of the plastic article causing it to shrink faster than the outside of the article resulting in sink marks and other blemishes on the outside of the article.
The prior art gases are under extremely high pressures and must accordingly be vented prior to opening of the mold in order to avoid explosion of the plastic article. Since the greatest amount of cooling and hardening occurs after the plastic article has been removed from the mold, the prior art gases can not be used to continuously exert outer pressure on the plastic article throughout the complete cooling and hardening of the article. This results in unpredictable shrinkage deformation in the walls and any ribs in the plastic article and makes it extremely difficult to control the formation of plastic articles having walls that are of different thicknesses.
The chilling effect of the high pressure expanded gases of the prior art also makes it difficult to avoid high stress knit lines where the plastic material flows together from separate directions in a mold cavity. These knit lines are caused by the flow characteristics of the plastic material in which the center of a flow travels much faster than the sides of a flow. When two of these plastic material flows combine, a knit line is formed which is under a great deal of stress. The chilling effect of the prior art gases actually inhibits the flow of the plastic material and cannot, therefore, be used to counter these knit lines by squaring off the head of the flow.
It is also known in the prior art to use a liquid blowing agent to form a plastic article having an inner honeycomb-like structure of interconnected small voids. The blowing agent is introduced in a direction transverse to the flow of the plastic material. This turbulent introduction of the blowing agent causes the formation of the small voids and is not, therefore, capable of forming a hollow plastic article.
The difficulties encountered in the prior art discussed hereinabove are substantially eliminated by the present invention.